percpu, locking: Revert ("percpu: Replace smp_read_barrier_depends() with lockless_dereference()")

lockless_dereference() is planned to grow a sanity check to ensure
that the input parameter is a pointer.  __ref_is_percpu() passes in an
unsinged long value which is a combination of a pointer and a flag.
While it can be casted to a pointer lvalue, the casting looks messy
and it's a special case anyway.  Let's revert back to open-coding
READ_ONCE() and explicit barrier.

This doesn't cause any functional changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Pranith Kumar <bobby.prani@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: kernel-team@fb.com
Link: http://lkml.kernel.org/g/20160522185040.GA23664@p183.telecom.by
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/percpu-refcount.h

@@ -136,14 +136,12 @@ static inline bool __ref_is_percpu(struct percpu_ref *ref,
 	 * used as a pointer.  If the compiler generates a separate fetch
 	 * when using it as a pointer, __PERCPU_REF_ATOMIC may be set in
 	 * between contaminating the pointer value, meaning that
-	 * ACCESS_ONCE() is required when fetching it.
-	 *
-	 * Also, we need a data dependency barrier to be paired with
-	 * smp_store_release() in __percpu_ref_switch_to_percpu().
-	 *
-	 * Use lockless deref which contains both.
+	 * READ_ONCE() is required when fetching it.
 	 */
-	percpu_ptr = lockless_dereference(ref->percpu_count_ptr);
+	percpu_ptr = READ_ONCE(ref->percpu_count_ptr);
+
+	/* paired with smp_store_release() in __percpu_ref_switch_to_percpu() */
+	smp_read_barrier_depends();
 
 	/*
 	 * Theoretically, the following could test just ATOMIC; however,